1 Topic 4: Physical Layer - Chapter 10: Transmission Efficiency Business Data Communications, 4e

2 Transmission Efficiency: Multiplexing  Several data sources share a common transmission medium simultaneously  Line sharing saves transmission costs  Higher data rates mean more cost- effective transmissions  Takes advantage of the fact that most individual data sources require relatively low data rates

3 Multiplexing Diagram

4 Alternate Approaches to Terminal Support  Direct point-to-point links  Multidrop line  Multiplexer  Integrated MUX function in host

5 Direct Point-to-Point

6 Multidrop Line

7 Multiplexer

8 Integrated MUX in Host

9 Frequency Division Multiplexing  Requires analog signaling & transmission  Total bandwidth = sum of input bandwidths + guardbands  Modulates signals so that each occupies a different frequency band  Standard for radio broadcasting, analog telephone network, and television (broadcast, cable, & satellite)

10 Frequency Division Multiplexing (FDM)

11 FDM Example: ADSL  ADSL uses frequency-division modulation (FDM) to exploit the 1-MHz capacity of twisted pair.  There are three elements of the ADSL strategy Reserve lowest 25 kHz for voice, known as POTS (Plain old telephone service) Use echo cancellation or FDM to allocate a small upstream band and a larger downstream band Use FDM within the upstream and downstream bands, using “discrete multitone” POTS UpstreamDownstream

12 DSL Modems Upload Speed: Kbps Download Speed: Mbps

13 Discrete Multitone (DMT)  Uses multiple carrier signals at different frequencies, sending some of the bits on each channel.  Transmission band (upstream or downstream) is divided into a number of 4-kHz subchannels.  Modem sends out test signals on each subchannel to determine the signal to noise ratio; it then assigns more bits to better quality channels and fewer bits to poorer quality channels. Frequency Bits/Hertz

14 Synchronous Time-Division Multiplexing (TDM)  Used in digital transmission  Requires data rate of the medium to exceed data rate of signals to be transmitted  Signals “take turns” over medium  Slices of data are organized into frames  Used in the modern digital telephone system US, Canada, Japan: DS-0, DS-1 (T-1), DS-3 (T-3),... Europe, elsewhere: E-1, E3, …

15 TDM

16 *SONET/SDH  SONET (Synchronous Optical Network) is an optical transmission interface proposed by BellCore and standardized by ANSI.  Synchronous Digital Hierarchy (SDH), a compatible version, has been published by ITU-T  Specifications for taking advantage of the high-speed digital transmission capability of optical fiber.

17 *SONET/SDH Signal Hierarchy

18 *STS-1 and STM-N Frames

19 Statistical Time Division Multiplexing (STDM)  “Intelligent” TDM  Data rate capacity required is well below the sum of connected capacity  Digital only, because it requires more complex framing of data  Widely used for remote communications with multiple terminals

20 STDM

21 STDM: Cable Modems  Cable TV provider dedicates two channels, one for each direction.  Channels are shared by subscribers, so some method for allocating capacity is needed--typically statistical TDM

22 Cable Modems Upload Speed: 400 Kbps Download Speed: Mbps

23 Cable Modem Scheme

24 *Transmission Efficiency: Data Compression  Reduces the size of data files to move more information with fewer bits  Used for transmission and for storage  Combines w/ multiplexing to increase efficiency  Works on the principle of eliminating redundancy  Codes are substituted for compressed portions of data  Lossless: reconstituted data is identical to original (ZIP, GIF)  Lossy: reconstituted data is only “perceptually equivalent” (JPEG, MPEG)

25 *Run Length Encoding  Replace long string of anything with flag, character, and count  Used in GIF to compress long stretches of unchanged color, in fax transmissions to transmit blocks of white space

26 *Run-Length Encoding Example

27 Huffman Encoding  Length of each character code based on statistical frequency in text  Tree-based dictionary of characters  Encoding is the string of symbols on each branch followed. String Encoding TEA SEA TEN

28 Lempel-Ziv Encoding  Used in V.42 bis, ZIP  buffer strings at transmitter and receiver  replace strings with pointer to location of previous occurrence  algorithm creates a tree-based dictionary of character strings

29 Lempel-Ziv Example

30 *Video Compression  Requires high compression levels  Three common standards used: M-JPEG ITU-T H.261 MPEG

31 *MPEG Processing Steps  Preliminary scaling and color conversion  Color subsampling  Discrete cosine transformation (DCT)  Quantization  Run-length encoding  Huffman coding  Interframe compression